Organic-inorganic hybrid lead halides as absorbers in perovskite solar cells: a debate on ferroelectricity

New photovoltaic materials have been extensively searched for in the last decade for clean and renewable solar energy conversion. Among them, the organic-inorganic hybrid lead halide perovskite solar cell is of current interest to the solar cell community because of its low processing cost, ease of fabrication and high power conversion efficiency. These perovskites show great potential to be photovoltaic absorbers with excellent optoelectronic properties, for example, high absorption coefficient, long charge carrier diffusion length, moderate ambipolar mobilities, tunable bandgap, etc. The unique physical and chemical properties of these materials are based on the crystal structure of the perovskites, named after the mineral form of CaTiO3. Most inorganic materials with a perovskite crystal structure typically show ferroelectric properties. Ferroelectricity in methylammonium lead tri-iodide (MAPbI(3)), an absorbing material in perovskite solar cells with a perovskite crystal structure, is now a topic of debate. Some theoretical and experimental investigations have shown the presence of a ferroelectric domain in these perovskites, although a lot of controversial results and conclusions also exist in the literature. In this review, notable signs of progress in the theoretical and experimental studies regarding ferroelectricity in organic-inorganic hybrid perovskites are summarized. In addition, future improvements of ferroelectric perovskites are proposed, paving the way towards prospective high-performance flexible and wearable ferroelectric solar cells and optoelectronic devices.